G·A·B·B·A

Regulated protein degradation mediated by the ubiquitin-proteasome system (UPS) is critical to eukaryotic protein homeostasis. Often vital to degradation of protein substrates is their disassembly, unfolding, or extraction from membranes. These processes are catalyzed by the conserved AAA-ATPase Cdc48 (also known as p97). Here, we characterize the Cuz1 (Cdc48-associated UBL/zinc-finger-1) protein, encoded by a previously uncharacterized arsenite-inducible gene in budding
yeast. Cuz1, like its human ortholog ZFAND1, has both an AN1 type zinc finger (Zf_AN1) and a divergent ubiquitin-like domain (UBL). We show that Cuz1 modulates Cdc48 function in the UPS. The two proteins directly interact, and the Cuz1 UBL, but not Zf_AN1, is necessary for binding to the Cdc48 N-terminal domain. Cuz1 also associates, albeit more weakly, with the proteasome, and the UBL is dispensable for this interaction. Cuz1-proteasome interaction is strongly enhanced by exposure of cells to the environmental toxin arsenite, and in a proteasome mutant, loss of Cuz1 enhances arsenite sensitivity. Whereas loss of Cuz1 alone causes only minor UPS degradation defects, its combination with mutations in the Cdc48-Npl4-Ufd1 complex leads to substantial further impairment. Cuz1 helps limit the accumulation of ubiquitin
conjugates on both the proteasome and Cdc48, suggesting a possible role in the transfer of ubiquitinated substrates from Cdc48 to the proteasome or in their release from these complexes.